The present description relates, in general, to internal combustion engines and, more particularly, internal combustion engines having variable valve operating characteristics.
Internal combustion engines use a timed coordination between intake and exhaust valves and a reciprocating piston within each cylinder to control the flow of intake charge into the cylinder and the flow of combustion exhaust gases from the cylinder.
In the past, the amount of movement of the intake and exhaust valves with respect to the intake and exhaust combustion chamber ports was fixed. Recent engine developments have allowed for variable valve characteristics in response to varying engine load, speed and other operating conditions.
Generally, the variable valve operating mechanism changes the lift, timing or operating angle of the valve. For example, such a mechanism can allow valve actuation from a low lift state at low engine speed or loads to a higher lift for greater valve stroke distance into the engine cylinder and a greater port opening at higher operating speeds and/or loads to allow for increased intake charge flow into the cylinder and to achieve increased engine efficiency.
However, in a high valve lift mode of operation, the clearance between the valve and the piston decreases. During an engine over-rev, which can be caused by miss-shifting, this causes valve surge in which the valves cannot strictly follow the cam profile, particularly during valve closing. Generally speaking, valve surge or float occurs when the valve springs does not have sufficient force to follow a valve closing cam profile. The resulting contact between the valve and the piston can cause catastrophic engine failure.
It would be desirable to provide a variable valve operating control method and apparatus which minimizes the possibility of valve to piston contact.